At the intersection near work are two juice shops. Right next to each other. Both serve the exact same thing: delicious, absolutely fresh juice. There are only two major differences between the shops. The one on the left is slightly cheaper. And it is Christian-owned.
Religion in Egypt is a particularly touchy subject. While the country is predominantly Muslim, there is a sizable Christian minority (known as Copts and are about 15 percent of the population) who have many complaints about the way they are treated by the government and by their fellow Egyptians. For background and some of the incidents that have occurred just this year, enjoy reading the US State Department's 2009 Report on Religion in Egypt. One fun fact from the report: There are approximately 125 Jews living in Egypt. To summarize, discrimination and persecution of Copts is rampant in Egypt. The most recent egregious widespread assault on Copts occurred this past spring when the government ordered all the pigs in Egypt to be killed to prevent swine flu from spreading. Muslims do not eat pork and Copts are the main pig farmers, and since swine flu isn't spread through pigs, Copts saw this as an attack on them.
People in Egypt literally wear religion on their sleeves. Most Copts have tattooed crosses on their right wrists. [One time in Coptic Cairo, I saw an old man with a rusty old ink machine and needle giving tattoos to people in a line. This is why Hepatitis C is so prevalent in the Coptic community.] Coptic women do not wear the hijab, or veil. There also potentially may be one biologically distinguishable characteristic but I only have anecdotal evidence. On my first day of college, I was talking to a guy before a class was starting. He told me he was from Greece and I said that I was Egyptian. He then paused for a bit before asking, "Are you Coptic?" Surprised, I said, "Yes. How did you know?" He, with a completely straight face, replied, "I looked into your eyes. Copts have lighter eyes."
Almost all Muslim women have at least their hair covered (say over 80%). This was drastically different during my parent's generation. A large number of Muslim men have callouses on their foreheads known as a zebiba, or raisin. Because Muslims are required to pray five times a day, which involves kneeling and putting your forehead to the ground, the bump can develop over time (if you want it to). Some Muslims believe that the more pious you are, the bigger your zebiba will be. I've seen some pretty gnarly ones.
You may also be able to tell a person's religion by their language. The most common greeting in the Middle East, Salam Alekum, and its response Wa Alekum Al Salam, is typically used by Muslims. Copts rarely use it as a greeting but go for other secular choices like Sabah el Khair or Masaa' el Khair. Also, it's common for people to say sentences and include the term Wallah or Walnaby or Waladra (And God, And the Prophet, And the Mother). The first one is used by all Egyptians; the second, only by Muslims; the third, only by Christians.
Finally, if you couldn't tell a person's religion from how they look or speak, you'll probably get it right away from his/her name. I have been in many cabs, talking with the drivers and midway through the ride, I'll get the test: "What's your name?" It means more than just, "Oh I'm just being friendly with you." It's a question of whose side I'm on. A Coptic driver will almost always say Al hamdu lilah, or praise be to God; a Muslim driver will usually say Oh... and then there'll be an awkward silence in the car at which point I always say something like, "But I don't hate Muslims and in America, they think we're all the same anyway and are generally scared of Arabs so we got to stick together man." A few cab drivers I've had will just come out and ask, "Thank Allah, you're Muslim, right?" which also gets awkward when I say No.
So, two juice guys right next to each other is more than just the (not so) free market at work. In Egypt, it's very easy to know which stores are Christian-owned and which are Muslim-owned because all the employees are usually from one religion. Also, Christian stores have pictures of saints in the window or on the wall while Muslim stores usually have Koranic verses hanging in the shop. I almost always go to the Coptic juice guys--partially because they're Coptic but also because they're cheaper.
Every day I get mango juice (plastic cup included) for 2 LE or 36 cents. Amazing. The other guys sell their juice for 2.25 LE and a plastic cup is an extra .25 LE. Absurd. Also, since I, along with the rest of the office, have been going to the Coptic juice guys every day, I've developed a rapport with them (Minaaaaa! Ya Monmon! Ezzayak?).